1. Field of the Invention
The present invention relates to a method for automatically setting a transmit aperture and a transmit apodization of an ultrasound transducer array.
2. Description of the Prior Art
Ultrasonic imaging systems use sound waves having frequencies greater than the audible range and generally include sound waves having frequencies in excess of 15-20 kHz. These systems operate on the pulse-echo principle that is used in SONAR and radar. In general, ultrasonic pulses are transmitted toward a target and echoes of the transmitted pulses bounce back from the target. The received echoes used to determine an image of the target.
There are a number of modes in which an ultrasonic transducer operates. The basic modes are A-mode, B-mode, M-mode and 2D-mode. The A-mode is amplitude mode in which signals are displayed as spikes that are dependent on the amplitude of the returning sound energy. The B-mode is a brightness mode in which the signals are displayed as various points whose brightness depends on the amplitude of the returning sound energy. The M-mode is a motion mode in which the B-mode is applied and a recorder such as a strip chart recorder allows visualization of a structure as a function of depth and time. The 2D-mode is two-dimensional imaging mode where the B-Mode is spatially applied by sweeping the beam so that structures are seen as a function of depth and width.
Ultrasonic imaging systems may be used to observe internal organs, tissues and vessels of a patient using a variety of the above-described imaging modalities. For example, B-mode scanning may be used to image tissues by portraying the tissues in a gray scale in which the brightness of a region is a function of the intensity of the ultrasound returns from corresponding regions of the tissues. In addition, Doppler scanning may be used to show the velocity of moving sound reflectors, such as blood flowing through an artery or vein.
A diagnostic ultrasound imaging system 10 is shown in FIG. 1. The ultrasound imaging system 10 includes a scanhead 20 having a transducer face that is placed in contact with a target area containing tissues, organs, or blood vessels of interest. The images of the ultrasonic pulses may be focused at preselected depths. There are two different relevant types of focus: receive focus and transmit focus. As explained below, the scanhead 20 includes an array of transducer elements 24, each of which transforms a transmit signal into a component of an ultrasound beam and transforms an ultrasound reflection into a respective receive signal. These signals are coupled between the scanhead 20 and an imaging unit 30 through a cable 26. The imaging unit is shown mounted on a cart 34. The imaging system also includes a control panel 38 for allowing a user to interface with the system 10. A display monitor 40 having a viewing screen 44 is placed on an upper surface of the imaging unit 30.
During operation, the transducer elements 24 in the scanhead 20 collectively transmit a beam 50 of ultrasound energy as shown in FIG. 2. Respective electrical signals, typically at a frequency of 1-20 MHz, are applied to all or some of the transducer elements 24. The number of transducer elements 24 to which electrical signals are applied determines the size of the transmit aperture. The size of the aperture affects the size of the imaging field and resolution, as explained below. In practice, the phases of the electrical signals applied to the transducer elements 24 are adjusted so that the beam 50 is focused in a focal position 52. The depth of the focal position 52 beneath the transducer face is controlled by the magnitude of the differences in phase of the electrical signals applied to the transducer elements 24. The focal length (or depth of field), which corresponds to the effective length of the focal position 52, is determined by the size and gain of the transmit aperture, i.e., the number of transducer elements 24 used to form the beam 50. The focal position 52 should ideally be positioned where features of maximum interest are located so that these features will be in the best attainable focus. The focal position 52 is shown in FIG. 2 as being considerably xe2x80x9csharperxe2x80x9d than is typical in practice when used with human tissue. The ultrasound from the individual transducer elements 24 is diffracted so that the effective length of the focal position 52 is actually more of an area where the beam is narrowed rather than a location where the beam converges at one point. It is also a common practice to transmit multiple focal positions or zones along a single line to extend this area where the beam is narrowed.
As previously mentioned, the transducer elements are also used to receive ultrasound reflections and generate corresponding electrical signals. As shown in FIG. 3, the phase and gain of the received signals are also adjusted to effectively generate a receive beam 56 that is focused to a focal position 58 corresponding to the phase differences between the signals coupled from the transducer elements 24. (In the interest of clarity in FIG. 3, beam components for only two transducer elements 24 are shown, although it will be understood that beam components would exist for all active transducer elements). The receive beam can also be xe2x80x9csteeredxe2x80x9d, i.e., offset from an axis that is perpendicular to the transducer face, by adjusting the phase differences between the signals coupled from the transducer elements 24. In practice, the phase differences between these signals are adjusted as a function of time delay from each ultrasound transmission so that the focal position 58 dynamically varies with depth from a relatively deep position 60 to a relatively shallow position 62 from where the ultrasound is reflected. As explained below, the disclosed invention relates to the locations of the focal position 52 for the transmit beam 50 rather than the locations of the focal position 58 for the receive beam 56.
It is an object of the present invention to provide a device and method for adaptive control of the focal zone or focal zones of an ultrasonic transducer array in response to user input defining a range of interest by automatically determining the depth range for each focal zone and the transmit apodization.
According to the present invention, the ultrasound system allows a user to control the focal zones or zones of a transducer array by inputting a point of interest and a range of interest. In a first embodiment, the system includes a first input parameter for the point of interest, a second input parameter for the top of the range of interest and a third input parameter for the bottom of the range of interest. A second embodiment assumes that the point of interest is always in the center of the range of interest and includes a range of interest control and a point of interest control. In the second embodiment, the range of interest control defines a total range (i.e., the distance between the top and bottom of the range). The system receives these control parameters and determines the actual focal zones required by the beamformer to achieve the desired control. For example, an algorithm may be used to select actual focal zones in response to the range of interest. The actual focal zones selected may optionally be shown to the user. Furthermore, the user may also alter the selected focal zones.
In a more detailed embodiment, other user preferences such as desired image quality and/or frame rate may be used to determine the number of zones to be used to fill the range of interest (i.e., zone spacing) and the size of the transmit apertures (which determines the depth of field for a single zone).